1. Field of the Invention
The present invention relates to a flaw detection testing method applied to a slot dovetail portion of a turbine generator rotor shaft.
2. Description of the Related Art
While a turbine generator rotor is in operation, a fretting fatigue fracture may occur in a slot dovetail portion of a turbine generator rotor shaft, that is, a shaft side of the vicinity of a shoulder portion of a rotor coil wedge to be inserted.
Known methods for detecting the fretting fatigue fracture during a periodic inspection are magnetic particle testing and eddy current testing, in which surface flaw detection is performed with an end ring and the rotor coil wedge disassembled.
However, to allow the end ring and the rotor coil wedge to be disassembled, the rotor needs to be carried to a manufacturing plant. This increases a period during which a power plant needs to be shut down, placing a heavy burden on power companies. Thus, ultrasonic testing is often adopted, which is an internal flaw detection technique that can be performed at the site without the need for disassembling.
When ultrasonic testing is applied to the slot dovetail portion, the slot angle of the slot dovetail portion to an outer peripheral surface of the rotor shaft, which is a test surface, needs to be taken into account.
The slot angle is known if a manufacturing drawing of the rotor shaft 1 is available. Otherwise, the slot angle needs to be pre-measured in order to allow an ultrasonic probe to be selected.
Furthermore, to increase the accuracy of detection of flaws such as the fretting fatigue fracture, which generally have small opening widths and which are directional, a reflection angle at which an ultrasonic beam is reflected by the flaw needs to be adjusted so as to avoid mode conversion losses in spite of the constraint on the slot angle.
If any flaw is detected, the depth of the flaw needs to be measured in order to determine whether operation can be continued or repairs are required.